In third generation (3G) mobile systems, such as W-CDMA, the downlink communication is based on a pseudonoise or scrambling code characterizing the different possible base stations. For the purpose of initiating any communication with a base station, any mobile station has to perform a so-called Cell search procedure for the purpose of searching for a cell and synchronising to its scrambling code.
The cell search is based on the use of three downlink channels transmitted by any base station, that is to say a Primary Synchronization Channel (P-SCH) a Secondary Synchronization Channel (S-SCH) and a Common Pilot Channel (CPICH). The P-SCH and the S-SCH channels are referred as the Synchronization channels.
Document, “Correlation properties of W-CDMA synchronization codes”, by Igor S. Simic and V. Popovic , recalls the structure of synchronization channel as well as the Cell search procedure which is based on the three following steps:
1) Slot synchronisation;
2) Frame synchronisation and code-group identification of the base station;
3) Scrambling-code identification.
The first step of the cell search procedure achieves slot synchronisation of the mobile station to the strongest base station based on the use of the primary SCH signal.
The second step of the cell search procedure is based on the use of the secondary SCH to find frame synchronisation and identify the code group of the base station found in the first step.
In the third step, the mobile determines the scrambling code used by the found base station, which is identified through correlation over the CPICH with all scrambling codes within the code group identified in the second step.
It should be noticed the second and third steps are particularly energy consuming and consequently, the occurrence of false detection in a mobile which is powered by a battery shows critical impact on the life of the battery.
As mentioned above, during the first step of the cell search procedure the mobile station uses the primary SCH to acquire slot synchronization. UMTS Primary Synchronization Channel (P-SCH, see in particular 3GPP TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD)”, v. 5.8.0, January 2005) uses Golay codes for their interesting correlation properties in case of frequency offset being the aim of this control channel to ensure User-Equipment (UE) coarse time and frequency synchronization (i.e. to acquire the slot timing and to acquire the carrier frequency within the range of [1 KHz, 15 KHz]).
Unfortunately, as a drawback, the Golay correlation shape has important side-lobes that, in particular in good channel conditions (line-of-sight or unloaded cell), can grow far above the noise floor.
Typical P-SCH detection algorithms make use of the cascade of:
1) Golay filtering and accumulation step, a procedure computing the correlation of the received signal with respect to known Golay code for all chip timing within a UMTS slot (2560 chips)
2) Constant-False-Alarm-Rate (CFAR) selection, a procedure consisting in applying to the correlation values a threshold function of the estimate of the noise variance and of a target False-Alarm-Rate (FAR).
As a consequence, there clearly exists a trade-off between the noise selectivity (low FAR) and the P-SCH signal detection probability: the least the false alarms due to noise peaks, the least the true P-SCH peaks detection probability is.
The combination of these two sources of un-wanted detections (Golay side-lobes and noise peaks) can result in a large amount of detections—possibly including false detection—that need to be further processed by the common UMTS cell-search and synchronization physical procedure consisting in:                Secondary-Synchronization channel, to acquire scrambling code group.        A Scrambling Code Search based on Common Pilot Channel (CPICH), to determine which primary scrambling code within the group is in use for searched cell.        A path detection based on CPICH, to determine the most relevant timing of wireless multi-path channel profile.        
As P-SCH detection procedure triggers of these successive searches, it is evident that false-alarms constitute the cause of energy waste especially for idle-mode UE operations and result in a considerable shortening of battery life.
It should be further noticed that the cell search procedures need to be considered not only during the initial cell search but also during searched in idle mode which is required for the purpose of continuously updating the list of the cells located in the neighborhood of the serving cells, and possibly useful for a handover.
In such a context, there is a strong desire to reduce the overall quantity of false alarms while keeping the probability of detection of true P-SCH peaks within a given confidence level.